The present invention relates generally to once through steam generators for nuclear power plants, and in particular to a trap for preventing feedwater debris from entering the space between the section of tubes positioned in the economizer which is integrally contained in the steam generator pressure vessel.
The pressurized steam generators, or heat exchangers, associated with nuclear power stations transfer the reactor-produced heat from the primary coolant to the secondary coolant, which in turn drives the plant turbines. Many of these steam generator pressure vessels have integral economizers wherein subcooled feedwater is directly channeled into the lower section of the tube bundle. The pressure vessels may about 75 feet long and have an outside diameter of about 12 feet. Within one of these steam generator pressure vessels, straight tubes, through which the primary coolant flows, may be typically ⅝ inch in outside diameter, but have an effective length of 52 feet or longer between the tube-end mountings and the opposing faces of the tubesheets. Typically, there may be a bundle of more than 15,000 tubes in one of these steam generator vessels. Debris from the feedwater system, transported by the feedwater flow, can become lodged between the tubes of the tube bundle and cause wearing and/or denting of the tubes.
While steam generator pressure vessels having integral economizers are known, as exemplified by U.S. Pat. No. 3,356,135 issued to Robert. K. Sayre; U.S. Pat. No. 3,547,084 issued to Theodore S. Sprague; and U.S. Pat. No. 3,771,497 issued to Theodore Sprague et al.; none of these prior art patents provide a debris trap within the steam generator pressure vessel to capture and prevent debris carried by the feedwater from entering the economizer tube section.
For a general description of the characteristics of nuclear steam generators, the reader is referred to Chapter 48 of Steam/its generation and use, 41st Edition, Kitto and Stultz, Eds., © 2005 The Babcock & Wilcox Company, Barberton, Ohio, U.S.A., the text of which is hereby incorporated by reference as though fully set forth herein.